Role Referral allows the assignment of roles to anonymous users who enter the site at a specific URL ([drupalsite]/referred) from a specified referring URL.
This module was originally developed to address a specific feature request by a client of Kosada Incorporated (a small Columbus Ohio-based consulting firm). Its primary intended use is in conjunction with an access control module (such as Taxonomy Access Control Lite, for instance), to make certain content hidden to the general public but immediately visible to anonymous visitors from certain pages (such as from the member-only section of the website of a partner organization) without them needing to first go through the hassle of creating an account. (Please note that HTTP Referer headers can be easily spoofed, so such a method should be considered only a deterrent to unauthorized access, and should certainly not be relied upon to secure critical information.)
Portions of the code were modeled after Jon Francis's ipAuthenticator module.